I'm fairly new to quantum computation and quantum complexity theory, but I came across some articles that suggest that quantum RAM (QRAM) is not very realistic assumption. For example some works show that even if we are willing to assume QRAM, the structure needed to implement it could also be exploited classically, preventing speedup, e.g., .
The reason why I was interested in QRAM is that I was wondering how fast one can compute the rank of an exponential size matrix (also if someone has any idea whether one can get any quantum speedup here I would be happy to hear!). However, the typical assumption for HHL-type algorithms seems to be that one can access the input implicitly, such as through a black-box that finds the nonzero entries in a given row of the matrix. Or sometimes it is assumed that the input is provided using QRAM.
Given this, is QRAM a self-consistent and interesting model of computation? Does it makes sense to even consider HHL-type algorithms with QRAM access?